Explain how the small intestine is adapted to absorb the products of digestion efficiently.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The small intestine is highly specialised for absorption:
1. It is lined with villi, finger‑like projections that increase the surface area by about 600‑fold.
2. Each villus contains a dense capillary network (and a lacteal) that draws nutrients into the blood and lymph, keeping a concentration gradient.
3. Soluble products such as glucose and amino acids diffuse across the villous epithelium into the capillaries.
4. Fatty acids and glycerol are taken up by enterocytes and then transported into the lacteals, the lymph vessels within the villi.
1. It is lined with villi, finger‑like projections that increase the surface area by about 600‑fold.
2. Each villus contains a dense capillary network (and a lacteal) that draws nutrients into the blood and lymph, keeping a concentration gradient.
3. Soluble products such as glucose and amino acids diffuse across the villous epithelium into the capillaries.
4. Fatty acids and glycerol are taken up by enterocytes and then transported into the lacteals, the lymph vessels within the villi.
Examiner tips
- Use the term ‘villi’ and ‘lacteals’ – examiners look for specific anatomical names.
- Show the sequence: villi → capillaries/lacteals → diffusion of soluble nutrients, lymphatic uptake of fats.
- Mention the concentration gradient to justify why absorption is efficient.
Common mistakes
- Confusing villi with microvilli – only villi are required for the question.
- Forgetting that fats enter the lymph via lacteals rather than capillaries.
- Not stating that diffusion is the mechanism for soluble nutrients.
Mark scheme (4 marks)
- The small intestine has villi (finger-like projections) which increase the surface area
- Villi have a rich blood supply / capillary network to carry absorbed nutrients away, maintaining a concentration gradient
- Soluble products (glucose / amino acids) are absorbed by diffusion into the blood
- Fatty acids and glycerol are absorbed into the lacteals (lymph vessels) inside the villi
Related
- All WJEC GCSE Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →